Automated data storage libraries are known for providing cost effective access to large quantities of stored data. Generally, data storage libraries include a large number of storage slots on which are stored portable data storage media. The typical portable data storage media is a tape cartridge or an optical cartridge. An accessing robot typically accesses the data storage media from the storage slots and delivers the accessed media to a data storage drive for reading and/or writing data on the accessed media. Suitable electronics both operate the accessing robot and operate the data storage drives to transmit and/or receive data from an attached on-line host computer system.
In a conventional automated data storage library, the storage slots are arranged in a planar orthogonal arrangement forming a "wall" of data storage media. The plane may be a flat plane, or may be a cylindrical plane. To double the storage capacity, two "walls" of data storage media may be provided on either side of the accessing robot.
A number of different companies manufacture automated data storage libraries today, each model displaying various different features. Some of the most expensive automated data storage libraries offer the capability of sharing the entire library with a plurality of hosts, such as the IBM 3494 Data Storage Library These automated data storage libraries have the internal capability to allocate storage slots and to handle contentions among the hosts for the same library components.
It would be advantageous in the lower cost environment to be able to share a library among multiple hosts and therefore amortize the cost of the library more widely. It is often difficult, however, to design components that can respond to multiple hosts. Frequently, this level of flexibility requires the addition of a cumbersome layer of supervisory software code in the library, which both increases the need for processing capability of the library and which can ultimately reduce the performance of other aspects of the library. Developing software for an automated data storage library to internally enable multiple incompatible hosts to manage a shared inventory may require a redesign of the library programming and may be sufficiently expensive as to be prohibitive for the low cost environment.
One example of a library sharing arrangement is the use of the multiple addresses for various components at connected ports having "relay drives", the multiple addresses comprising differing "logical unit numbers" in SCSI parlance so that a host sends robot accessor commands to a "relay" drive which forwards the commands over a loop or bus to a library controller. This approach requires modifications to the components of the automated data storage library. This approach is described in coassigned U.S. patent application Ser. No. 08/931,856, filed Sep. 16, 1997, Basham et al.
In the low cost environment where hosts and libraries alike are often mass produced devices, it would be advantageous to have a library sharing capability which does not require modification to any of the components of the mass produced automated data storage library.